PURPOSE: To decrease the number of coils and capacitors, and to attain miniaturization by using the leakage inductance of a noise cut transformer as one of the coils, and using a filter circuit constituted of the coils and the capacitors.

CONSTITUTION: A primary side coil 62 and a secondary side coil 63 are wound along a core 65 coaxially in parallel. Then, the filter circuit constituted of the coils and the capacitors is used by using a noise cut transformer 10a in which the insulation dielectric withstanding voltage of the primary and secondary sides is secured by an interval (t) between the two coils 62 and 63, and the leakage inductance of the noise cut transformer 10a as part of the coils. Then, an inductance 8 is decided according to the leakage inductance of the noise cut transformer 10a, and actually the peak of attenuation is moved by changing the capacity of a capacitor 9. Also, a CML relay 1 switches a telephone line 107 to a facsimile side and a handset 108 or slave telephone side.